In many laser applications, it is necessary to deliver a beam of radiation from the laser to a device remote from the laser that must have as many as six degrees of freedom of movement in space. Such applications include, in particular delivery, of a laser beam to a handpiece or applicator for applying the laser beam in a medical or dental treatment. A handpiece or applicator may include focusing optics for the beam or a scanning arrangement for scanning the beam over a treatment area. A common and convenient delivery arrangement for laser radiation having a wavelength in the visible or near infrared region of the electromagnetic spectrum is to transport the radiation (beam) from the laser to the handpiece via an optical fiber or a bundle of optical fibers.
Transmission via optical fibers is practically limited to radiation having a wavelength less than about 2600 nanometers (nm). At wavelengths longer than this, delivery is usually effected via what is generally referred to by practitioners of the art as an articulated arm. An articulated arm comprises a plurality of tubes joined one to another via one-axis or two axis rotatable joints or couplers. Internal mirrors in the couplers steer a beam through the arm from one tube to another, and along the length of the tubes.
FIG. 1 schematically illustrates one example 10 of a commercially available articulated arm supplied by LASER MECHANISMS™, INC. Farmington Hills, Mich. Arm 10 includes a launch housing 12 having a mounting plate 14, fixedly attachable to a laser (not shown). The laser beam enters the housing via an aperture 16 in plate 14, and is directed by a mirror LM into a first tube 18 of the arm. The beam is directed out of tube 18 by first mirror M1 housed in a joint or coupler 22 including mirrors M1 and M2 (not visible in the FIG. 1). Mirrors M1 and M2 direct the beam into and along a second tube 20. Coupler 22 between tubes 18 and 20 permits pivoting or rotation of the coupler and mirror M1 about the longitudinal axis of tube 18 as indicated by arrows R1, and permits pivoting of tube 20 about an axis perpendicular to the longitudinal axis of tube 20 (and tube 18) as indicated by arrows R2. Tube 20 is supported by an elongated support member 24 having a right angled bracket 26 thereon, to which is attached a counter weight 28.
Tube 20 is attached to a tube 30 by another coupler 32 including mirrors M3 and M4. Coupler 32 is rotatable about the longitudinal axis of tube 20 as indicated by arrows R3, and permits pivoting of tube 30 about an axis perpendicular to the longitudinal axis of tube 30 as indicated by arrows R4. The beam traveling along tube 20 is reflected via mirrors M3 and M4 into and along tube 30. A coupler 34, at an end of tube 30 includes a mirror M5 and a mirror M6 and is pivotable about the longitudinal axis of tube 30 as indicated by arrows R5 and about an axis perpendicular to the longitudinal axis of the tube 30 as indicated by arrows R6. Mirrors M5 and M6 in coupler 34 direct the beam from tube 30 through two right-angle bends in a direction perpendicular to the longitudinal axis of tube 30. A final coupler 36 includes a mirror M7 and is pivotable about the axis of the section emerging from coupler 36 as indicated by arrows R7. Coupler 36 has a flange 38 thereon from which the laser beam is delivered, and to which can be attached a handpiece or the like (not shown in FIG. 1) for focusing, shaping, dividing or scanning the beam. Such a handpiece, being attached to flange 38, would be pivotable as indicated by arrows R7.
In this type of arm, the two long arms 18 and 20 and rotations R1, R2 and R4 are primarily responsible for selection the position of the output end of the arm in a three dimensional working space or volume around the launch unit, definable in terms of X, Y, Z Cartesian axes. Three degrees of freedom of movement along these axes determine the position of the end of the arm in the working volume. The remaining rotations R3, R5, R6 and R7, cooperative with the other rotations, provide three additional degrees of freedom, i.e., rotation (pivoting) about the X-axis, pivoting about the Y-axis, and pivoting about the Z-axis.
Each mirror in an articulated arm is a source of energy loss in a beam as the mirrors are never exactly 100 percent reflective for the wavelength of the laser radiation. Even if each mirror has a reflectivity of 99%, the total energy loss from absorption in a seven mirror arm will be 7%. Further, no matter how well engineered coupler of the articulated arm may be, the couplers will never be completely free of play and accumulation of play at all of the joints can result in movement-sensitive changes in direction (pointing) of the beam as the beam leaves the articulated arm. Because of a requirement for freedom and smoothness of pivoting together with minimizing of free play in rotatable joints, the couplers are expensive and contribute to most of the cost of an articulated arm. The cost of such an arm is essentially proportional to the number of couplers therein. Clearly, it would be advantageous if the number of mirrors and couplers in an articulated arm could be reduced to reduce energy losses and beam pointing variations, and reduce the cost of the arm without giving up degrees of freedom of movement of a handpiece or the like attached to the arm. A typical handpiece at the end of the arm may require the laser beam to maintain an input tolerance of 10 to 100 micrometers (μm). For a one-meter-long arm, this means that the combined angular tolerance for all couplers combined must be less than 10 to 100 microradians.